1. Field of the Invention
The present invention relates to an array substrate for a liquid crystal display device and a method of fabricating the same, and more particularly, to an array substrate having an integrated driving circuit.
2. Discussion of the Related Art
In general, a liquid crystal display (LCD) device uses optical anisotropy and polarization properties of liquid crystal molecules to display images. The LCD device includes first and second substrates facing each other and a liquid crystal layer interposed therebetween. The first substrate, typically referred to as an array substrate, includes thin film transistors (TFTs) as switching elements. The second substrate, typically referred to as a color filter substrate, includes a color filter. The TFT includes a semiconductor layer of amorphous silicon or polycrystalline silicon. Since a process using the amorphous silicon is performed in a relatively low temperature and requires a relatively inexpensive insulating substrate, the amorphous silicon has been widely used in fabricating the TFTs. However, since the amorphous silicon has randomly arranged silicon atoms, the amorphous silicon has weak bonding strengths between the silicon atoms, dangling bonds, and low field effect mobility. Accordingly, the TFTs made from amorphous silicon are not adequate for a driving circuit.
By contrast, the polycrystalline silicon has excellent field effect mobility. Hence, the polycrystalline silicon is typically used for fabricating the TFTs of the driving circuit. Further, by forming the driving circuit on the substrate using the polycrystalline silicon without using tape automated bonding (TAB), the LCD device becomes compact, and production cost of the LCD device decreases.
FIG. 1 is a schematic plane view showing an array substrate of an LCD device according to the related art. As shown in FIG. 1, the first substrate 30 includes a display region D1 and a non-display region D2. A switching TFT (Ts) and a pixel electrode 17 connected to the switching TFT (Ts) are formed in a pixel region P. A gate line 12 along a first direction and a data line 14 are formed to define the pixel region P. The pixel regions P are formed as a matrix in the display region D1.
A gate driving circuit 16 and a data driving circuit 18 are formed in the non-display region D2 of the first substrate 30. The gate driving circuit 16 and the data driving circuit 18 supply control signals and data signals to the pixel regions P through the gate lines 12 and the data lines 14, respectively. The gate driving circuit 16 and the data driving circuit 18 have TFTs having a complementary metal-oxide semiconductor (CMOS) structure to apply a suitable signal to the respective pixel region P. The TFTs having CMOS structure are typically used for rapidly treating signals in the driving circuit. The CMOS structure includes a combination of n-type (i.e., negative type) and p-type (i.e., positive type) TFTs.
FIG. 2 is a schematic plane view showing a display region of an LCD device having an integrated driving (IC) circuit on the first substrate according to the related art. As shown in FIG. 2, the first substrate 30 includes the gate lines GL, the data lines DL, the switching TFTs Ts, the pixel electrodes 82, and storage capacitors Cst. The gate lines GL and the data lines DL cross each other to define the pixel regions P, and the switching TFTs Ts including a gate electrode 52, an active layer 38, and source and drain electrodes 74a and 74b are formed at a crossing portion of the gate and data lines GL and DL. The pixel electrode 82 connected to the drain electrode 74b and the storage capacitor Cst including first, second, and third storage electrodes 40, 54, and 76 are formed in the pixel region P. The storage capacitors Cst are connected to a storage line SL.
The gate driving circuit 16 and a data driving circuit 18 is formed in a non-display region D2 at a periphery of the display region D1. The gate driving circuit 16 and the data driving circuit 18 supply signals to the gate and data lines GL and DL, respectively. Since the gate driving circuit 16 and the data driving circuit 18 have low leakage currents and easily control the signals supplied to the gate and data lines GL and DL, the gate driving circuit 16 and the data driving circuit 18 include TFTs of n-type or TFTs with the CMOS structure.
FIG. 3 is a cross-sectional view of a driving circuit, and FIG. 4 is a cross-sectional view taken along the line IV-IV of FIG. 2. The driving circuit DC, which may be one of the gate driving circuit 16 and the data driving circuit 18, is formed in the non-display region D2 and includes TFTs with CMOS structure, i.e., a combination of a first TFT Tp of p-type and second TFT Tn of n-type, as shown in FIG. 3. As shown in FIG. 4, a switching TFT Ts as a switching element, a pixel electrode 82, and a storage capacitor Cst are formed in the display region D1. The TFT Ts is typically an n-type TFT. The storage capacitor Cst includes a first capacitor C1 and a second capacitor C2, which are connected to each other in series. The first capacitor C1 includes a first storage electrode 40 and a second storage electrode 54. The second capacitor C2 includes the second storage electrode 54 and a third storage electrode 76. A process for fabricating the LCD device according to the related art is performed by a nine mask process including several doping process for forming the storage capacitor Cst, the first TFT Tp, and the second TFT Tn.
FIGS. 5A to 5I are cross-sectional views showing processes of fabricating a driving circuit according to the related art. FIGS. 6A to 6I are cross-sectional views showing processes of fabricating a display region of an array substrate according to the related art. FIGS. 7A to 7I are cross-sectional views showing processes of fabricating a portion taken along the lines VII-VII of FIGS. 6A to 6I.
A first mask process will now be described with reference to FIG. 5A, FIG. 6A and FIG. 7A. As shown in FIG. 5A, FIG. 6A and FIG. 7A, a buffer layer 32 is formed on the first substrate 30 by depositing a first insulating material. The non-display region D2 having first and second areas A1 and A2 (FIG. 5A) and the display region D1 having third and fourth areas A3 and A4 in the pixel region P (FIG. 7A) are defined on the first substrate 30. Then, a first semiconductor layer 34 of polycrystalline silicon in the first area A1, a second semiconductor layer 36 of polycrystalline silicon in the second area A2, and a third semiconductor layer 38 of polycrystalline silicon in the third area A3 and the first storage electrode 40 of polycrystalline silicon in the fourth area A4 are formed on the buffer layer 32. The layers of polycrystalline silicon are formed by depositing amorphous silicon, crystallizing the amorphous silicon using a laser, and patterning the crystallized amorphous silicon by a first mask process. The first, second, and third semiconductor layers 34, 36, and 38 function as an active layer, and the first storage electrode 40 functions as an electrode of the first capacitor (C1 of FIG. 4). The third semiconductor layer 38 and the first storage electrode 40 may be integrated.
A second mask process for doping the first storage electrode 40 with impurities is described with reference to FIG. 5B, FIG. 6B, and FIG. 7B. As shown in FIG. 5B, FIG. 6B, and FIG. 7B, a first photoresist (PR) pattern 42 is formed on the first, second, and third semiconductor layers 34, 36, and 38 by coating and patterning a photoresist layer during a second mask process. The first PR pattern 42 covers the first, second, and third areas A1, A2, and A3 while exposing the first storage electrode 40 in the fourth area A4. Then, the first storage electrode 40 is doped with high concentration n-type impurities using the first PR pattern 42 as a doping mask. Since the first storage electrode 40 functions as the electrode of the first capacitor (C1 of FIG. 4), impurities are doped to decrease electrical resistance of the first storage electrode 40. Then, the first PR pattern 42 is removed.
A third mask process will now be described with reference to FIG. 5C, FIG. 6C, and FIG. 7C. As shown in FIG. 5C, FIG. 6C, and FIG. 7C, a gate insulating layer 46 is formed on the first, second, and third semiconductor layers 34, 36, and 38 and the first storage electrode 40 by depositing a first inorganic insulating material. The inorganic insulating material includes one of silicon nitride and silicon oxide. Then, a first gate electrode 48, a second gate electrode 50, a third gate electrode 52, and a second storage electrode 54 are formed on the gate insulating layer 46 by deposing and patterning a first conductive metal by the third mask process. The first gate electrode 48 corresponds to a center of the first semiconductor layer 34 and has a size smaller than the first semiconductor layer 34. The second gate electrode 50 corresponds to a center of the second semiconductor layer 36 and has a size smaller than the second semiconductor layer 36. The third gate electrode 52 corresponds to a center of the third semiconductor layer 38 and has a size smaller than the third semiconductor layer 38. The second storage electrode 54 corresponds to a center of the first storage electrode 40 and has substantially the same size as the first storage electrode 40. At the same time, the gate line GL extending from the third gate electrode 52 is formed along a side of the pixel region P (FIG. 6C). A storage line SL extending from the second storage electrode 54 is formed across the pixel region P (FIG. 6C).
A fourth mask process for doping the second and third semiconductor layers 36 and 38 with impurities is described with reference to FIG. 5D, FIG. 6D, and FIG. 7D. As shown in FIG. 5D, FIG. 6D, and FIG. 7D, a second PR pattern 56 is formed in the first area A1 by coating and patterning PR. The second PR pattern 56 covers the first semiconductor layer 34. Then, ends of the second semiconductor layer 36 and ends of the third semiconductor layer 38 are doped with high concentration n-type impurities using the second PR pattern, the second and third gate electrodes 50 and 52, and the second storage electrode 54 as a doping mask such that the ends of the second semiconductor layer 36 and ends of the third semiconductor layer 38 have an ohmic contact characteristic. Then, the second PR pattern 56 is removed.
A fifth mask process for doping the first semiconductor layer 34 with impurities will now be described with reference to FIG. 5E, FIG. 6E, and FIG. 7E. As shown in FIG. 5E, FIG. 6E, and FIG. 7E, a third PR pattern 58 is formed in the second, third, and fourth areas A2, A3, and A4 by coating and patterning a layer of photoresist. The third PR pattern 58 covers the second and third semiconductor layers 36 and 38 and the first storage electrode 40. Because the first storage electrode 40 is covered with the second storage electrode 54, the third PR pattern 58 in the fourth area A4 is not essential. Then, ends of the first semiconductor layer 34 are doped with high concentration p-type impurities using the third PR pattern 58 and the first gate electrode 48 as a doping mask such that the ends of the first semiconductor layer 34 has an ohmic contact characteristic. The third PR pattern 58 is then removed.
A sixth mask process will now be described with reference to FIG. 5F, FIG. 6F, and FIG. 7F. As shown in FIG. 5F, FIG. 6F, and FIG. 7F, an interlayer insulating layer 60 having first, second, third, fourth, fifth, and sixth contact holes 62a, 62b, 64a, 64b, 66a, and 66b is formed on the first, second, and third gate electrodes 48, 50, and 52, and the second storage electrode 54 by depositing and patterning a second inorganic insulating material. The inorganic insulating material includes one of silicon nitride and silicon oxide. The first and second contact holes 62a, 62b expose the ends of the first semiconductor layer 34, respectively. The third and fourth contact holes 64a and 64b expose the ends of the second semiconductor layer 36, respectively. The fifth and sixth contact holes 66a and 66b expose the ends of the third semiconductor layer 38, respectively.
A seventh mask process will now be described with reference to FIG. 5G, FIG. 6G, and FIG. 7G. First, second, and third source electrodes 70a, 72a, and 74a and first, second, and third drain electrodes 70b, 72b and 74b are formed on the interlayer insulating layer 60 by depositing and patterning a second conductive metal. The second conductive metal includes one of chrome, molybdenum, tungsten, copper, and aluminum alloy. The first source electrode 70a and the first drain electrode 70b contact the first semiconductor layer 34 through the first and second contact holes 62a and 62b, respectively. The second source electrode 72a and the second drain electrode 72b contact the second semiconductor layer 36 through the third and fourth contact holes 64a and 64b, respectively. The third source electrode 74a and the third drain electrode 74b contact the third semiconductor layer 38 through the fifth and sixth contact holes 64a and 64b, respectively. At the same time, a third storage electrode 76 extending from the third drain electrode 74b is formed in the fourth area A4. The second capacitor C2 includes of the second storage electrode 54 and the third storage electrode 76. The data line DL extending from the third source electrode 74a is formed along one side of the pixel region P and crosses the gate line GL to define the pixel region P.
Through the above-mentioned seven mask processes, the CMOS structure having a combination of the p-type TFT and the n-type TFT is formed in the non-display region D2, and the n-type switching TFT is formed in the third area A3 of the display region D1. Also, the first and second capacitors are formed in the fourth area A4 of the display region D1.
An eighth mask process is described with reference to FIG. 5H, FIG. 6H, and FIG. 7H. As shown in FIG. 5H, FIG. 6H, and FIG. 7H, a passivation layer 78 is formed on the source electrodes 70a, 72a, and 74a and drain electrodes 70b, 72b, and 74b by depositing and patterning a second insulating material. A drain contact hole 80 exposing one of the third drain electrode 74b and the third storage electrode 76 is formed in the passivation layer 78.
A ninth mask process will now be described with reference to FIG. 5I, FIG. 6I, and FIG. 7I. As shown in FIG. 5I, FIG. 6I, and FIG. 7I, a pixel electrode 82 is formed on the passivation layer 78 by depositing and patterning a transparent conductive metal. The transparent conductive metal includes one of indium-tin-oxide (ITO) and indium-zinc-oxide (IZO). The pixel electrode 82 contacts one of the third drain electrode 74b and the third storage electrode 76 through the drain contact hole 82.
Through the above-mentioned nine mask processes, the array substrate for the LCD device having the integrated driving circuit according to the related art is formed. However, due to the number of processes, processing time is delayed and production costs are increased. Also, since the array substrate is manufactured through so many processes, the defect rate increases.